1. Field of the Invention
The present invention relates to phase shift mask.
2. Description of the Background Art
When a semiconductor device is fabricated in a process, photoresist patterns and the like are formed in a large number of photolithography steps performing exposure through masks. For example, a photoreceptive circuit substrate is provided with a circuit diagram through exposure and then chemically processed to produce an integrated printed wired circuit.
In photolithography, it is preferable that fine patterns can be provided through exposure. Photolithography has its limit depending on the wavelength of light used to perform exposure. The limit can be reduced by phase shifting employing a Levenson mask. More specifically, a mask provided with a pattern to be transferred has a portion that shifts light's phase, or a shifter. Light having passed through the shifter and thus having its phase sifted and light that does not pass through the shifter and thus has its phase unshifted mutually interfere, which is utilized to provide increased resolution. The Levenson mask includes that having a shifter layer provided on the substrate's surface as well as that having as a shifter a portion dug into the substrate's main surface.
If a dug portion is provided with a Levenson mask, then the substrate's main surface that transmits light serves as a 0 portion that does not shift the light's phase and the dug portion serves as a π portion that shift the light's phase. The π portion is dug to a depth allowing a phase to be offset by one half of the wavelength.
The substrate's dug or π portion has a sidewall, which has an effect so that a dimension that is transmitted through the dug portion, and thus transferred is smaller than desired. To eliminate this effect, the dug portion is formed to be larger than an opening of a shielding film deposited on a surface of the Levenson mask. More specifically, if the shielding film has the opening surrounded by an eaves, the 0 and π portions transfer a dimension with reduced difference (i.e., the 0 portion's transferred dimension minus the π portion's transferred dimension is reduced). In contrast, if the eaves is excessively large, the shielding film can peel off and thus provide an accordingly changed geometry for projection, which can fatally damage a circuit that is formed. Accordingly there is a demand for forming an eaves having a size with precision.
The Levenson mask having a shielding film with an opening surrounded by an eaves can be formed by dry etching to form a dug portion in a rectangle as seen in cross section, and subsequently wet etching to increase the dug portion in size. More specifically, the wet etching can provide the shielding film with the eaves and provide the dug portion with an undercut. The eaves's size can be estimated from the wet etching's time. Alternatively, it can be measured with an atomic force microscope or similar contact-type equipment.
Furthermore, Japanese Patent Laying-Open No. 2003-344987 discloses a method of measuring a size of an eaves of a phase shift mask, including the steps of: forming on a substrate a light shielding pattern and a monitoring pattern dimensioned as prescribed; etching the substrate with the monitoring and light shielding patterns, as prescribed, to provide the substrate with a step; and removing the monitoring pattern from the stepped substrate and measuring, as a size of an eaves, a dimension of a plane of the substrate at a particular region located directly under the monitor pattern and defined by the step.
If the Levenson mask's eaves is measured by estimating its size from the wet etching's time, as described above, a large error is introduced between the eaves's actual and estimated sizes, and whether the eaves has a size as desired can also not directly be confirmed. Furthermore, if it is measured with contact-type equipment, a probe is used to trace a surface of a substance and a dug portion inside that provided with the eaves can hardly be measured accurately.
Thus in conventional art the Levenson mask's eaves cannot directly be measured accurately and whether the eaves's size has a desired value can only be determined by employing a completed Levenson mask for transfer and confirming a result thereof.
The Levenson mask disclosed in Japanese Patent Laying-Open No. 2003-344987 requires providing the substrate's main surface with a light shielding pattern as well as a monitoring pattern and, furthermore, subsequently removing the monitoring pattern alone, resulting in an increased number of steps to produce the Levenson mask.